Many current image sensors are composed of pixels containing multiple transistors, produced for example by means of CMOS or “Complementary Metal Oxide Semiconductor” technologies. CMOS technologies allow a high degree of integration to be obtained. For example, pixels are known comprising, in addition to a photosensitive element such as a diode, three transistors. These pixels are known as 3T pixels. One of the internal transistors of the pixel is used for resetting the charges accumulated in the photosensitive element, the second transistor is used as a follower and the third allows the readout of the pixel by connecting the output of the follower transistor to a column conductor. This type of pixel requires a reset command and a readout command. These two commands are generally conveyed by row conductors.
Certain devices employ alternative, less expensive, technologies based on thin-film deposition. The pixels forming such a device generally comprise a photosensitive element associated with an element fulfilling a switching function. The photosensitive element is for example formed by a diode, mounted in series with the switching element. The switching element can for example be a transistor or a diode, known as a switching diode, the “closed” or “on” state of which allows the charges accumulated in the photosensitive diode during illumination to be read out, and the “open” or “off” state of which allows the photosensitive diode to accumulate charges as a function of the illumination. The two diodes are mounted with opposite directions of conduction, in what is known as a “back-to-back” configuration.
These technologies implement thin-film field-effect transistors, or TFTs. In these thin-film techniques, many families are used. The transistors can be based on hydrogenated amorphous silicon (aSiH), polysilicon amorphous or crystalline indium, gallium, zinc oxide known by the abbreviation IGZO. Other families of transistors of TFT type can be implemented, such as for example organic TFTs.
Techniques for depositing thin semiconductor films on insulating substrates, of glass for example, allow matrices of photosensitive points to be produced which are able to generate an image from visible or near-visible radiation. In order to use these matrices in the detection of radiological images, a scintillating screen can be interposed between the X-ray radiation and the matrix to convert the X-ray radiation into light radiation in the wavelength band to which the pixels are sensitive.
In contrast to 3T pixels, the alternative technologies only have a single switching element. They only need a single control row per pixel, which is the reason for their simplicity. When the switching element is in the on state, the charges accumulated in the photosensitive element are transferred by a column conductor to a readout circuit. At the level of the pixel, there is no reset command. The transfer of the charges fulfils the function of resetting the pixel. In other words, the readout and reset operations are simultaneous.
A given pixel is only addressed once per frame. Consequently, its integration time is equal to the duration of the frame.
Control of the integration time can be achieved externally to the photosensitive matrix, for example by means of a shutter only allowing the incident illumination to reach the device for a fraction of the duration of the frame. In X-ray-based applications, the source is generally controlled. X-ray flashes are emitted according to the control of the frame. More generally, the exposure time of the photosensitive device is not controlled by the commands of the photosensitive matrix.
In video applications (capture of consecutive images) with control of the source (stroboscopy), even if the pixel in the integration state receives no light, the active element continues to generate noise. In the case of a photodiode, the dark current will be superposed over the useful signal, reducing the available dynamic range of the device.
If the incident radiation is continuous (no shutter), the disadvantage is the potential saturation of the pixel due to an excessive integration time in relation to the quantity of incident radiation. It is possible to reduce the integration time by increasing the speed of the frame. However, this reduction causes a decrease in the readout duration of each pixel and therefore requires an increase in the bandwidth. A photodiode behaves like a capacitor in which charges are stored during the integration time. When the quantity of charges exceeds a limit dependent on the bias of the photodiode, the surplus of charges can either flow into neighbouring pixels, generating crosstalk between pixels, or into the column conductor used for pixel readout by closing the switching element, generating an artefact in the image arising from the device in the form of a vertical line.
A control of the duration of integration independent of the duration of the frame may therefore prove to be useful in order to avoid these effects. This is possible in photosensitive devices of 3T type by means of the zeroing transistor but not in devices the pixels of which only comprise a single switching element.